Systems and methods for generating power that utilize the combustion of fossil fuel(s) with carbon dioxide as a working fluid are described in U.S. Pat. No. 8,596,075, which is incorporated herein by reference in its entirety. Estimates indicate that fossil fuel(s) will continue to provide the bulk of the world's electric power requirements for the next 100 years as non-carbon power sources are developed and deployed. Known methods of power generation through the combustion of hydrocarbon fuels, such as fossil fuel(s) and/or suitable biomass, however, are limited by rising energy costs and a desire to decrease production and emission of carbon dioxide (CO2). Global warming is increasingly viewed as a potentially catastrophic consequence of increased emissions of CO2 by developed and developing nations. Solar and wind power are probably incapable of replacing, in the near future, power generated from the combustion of fossil fuel(s) and/or other hydrocarbon fuels. Additionally, nuclear power has associated dangers, which include proliferation of nuclear materials and disposal of nuclear waste.
Power generated from combustion as noted above is now increasingly burdened with desires for capturing high pressure CO2 for delivery to sequestration sites, enhanced oil recovery operations, and/or general pipeline injection for reuse. This desire for capturing CO2 is difficult to fulfill with current power generation systems and methods, such as high efficiency combined cycle plants; the incurred parasitic load of capturing CO2 may result in very low thermal efficiencies. Moreover, capital costs are high for achieving the desired level of CO2 capture. These and other complications result in significantly higher electricity costs (e.g., an increase of as much as 50-70%) compared to systems that emit CO2 to the atmosphere. An increasingly warming planet and/or carbon emission taxation could catastrophically impact the environment and the economics of power generation. Accordingly, a need exists in the art for systems and methods that provide high efficiency power generation with a reduction in CO2 emission by capturing CO2, which may provide for lower electricity costs and improved ease of sequestering and storing captured CO2.
One approach to overcoming the thermodynamic burden of recapturing CO2 is a high efficiency power generation cycle that employs a substantially pure CO2 working fluid having pressures suitable for pipeline injection. This approach has gained increasing popularity, with designs employing recirculating trans-critical, supercritical, and/or ultra-supercritical working fluids. These working fluids, which primarily include oxy-combustion formed CO2, are maintained in operational windows that, at points within the power generation cycle, coincide with pressures and temperatures suitable for pipeline injection. At these coincidental points, CO2 may be safely vented from the power generation cycle to a pipeline and/or downstream reuse process that requires such a highly pressurized and purified CO2, while still maintaining high efficiencies within the power generation cycle.
One such power generation cycle may utilize the oxy-combustion of a hydrocarbon fuel to power a fully recuperated, trans-critical carbon dioxide working fluid in a Brayton-style power generation cycle, which is disclosed in previously mentioned U.S. Pat. No. 8,596,075. In various aspects, the power generation cycle inherently captures substantially 100% of the CO2 formed from the combustion of a hydrocarbon fuel that has a desired sequestration or pipeline pressure. Further, the captured CO2 has a substantially high purity. In aspects where natural gas is used as the combustible fuel, such a power generation cycle can achieve thermal efficiencies that are substantially equivalent to efficiencies obtained in general combined cycle systems without a reduction in the efficiency of capturing CO2 at pressures up to and beyond 300 bar. In particular, when the combustible fuel utilized contains low concentrations of sulfur and nitrogen, such as natural gas, the CO2 produced from the cycle may be vented to a CO2 pipeline at the required molar purities with little to no additional post-treatment steps.
Solid combustion fuels, such as coals of varying rank, pet-coke, bitumen or biomass, may contain elevated concentrations of sulfur, nitrogen, and other fuel derived impurities. When such fuels are utilized, they must first be gasified with substantially pure oxygen in a high pressure gasifier to produce a fuel gas. The fuel gas is then cleaned of any remaining particulate, cooled, compressed to the required combustion pressure, and then introduced to the combustor of the power generation cycle for oxy-fuel combustion. Additionally, sour natural gas containing elevated concentrations of sulfur-containing compounds can be utilized. Fuel derived impurities, such as sulfur and nitrogen containing compounds, are not removed from the fuel gas prior to oxidation. As such, the fuel gas retains substantial concentrations of impurities that may include H2S, COS, CS2, NH3, HCN, Hg, and other trace components depending on the primary fuel source.
Oxyfuel-type combustion of fuel gas produces a relatively pure CO2 stream, a quantity of water (H2O), and any residual post-combustion compounds, which may include molecular oxygen (O2). If an air separation unit is utilized in the power generation cycle, relatively low concentrations of molecular nitrogen (N2) and argon (Ar) may be present, with nitrogen also originating from any designed air ingressions. Additionally, other oxidation reactions of sulfur and nitrogen-containing compounds may occur with remaining oxidant, which may be intentionally maintained in excess. This oxidation may result in the formation of several impurities, derived from either the primary fuel or partial oxidation process and produced in the oxy-fuel combustor and/or other high temperature regions of the power generation cycle. Impurities may include sulfur oxides (SOx), such as sulfur dioxide (SO2) and sulfur trioxide (SO3), which form when fuel-derived sulfur is oxidized at high temperature. Other impurities may include nitrogen oxides (NOx), such as nitrogen oxide (NO) and nitrogen dioxide (NO2), which form primarily when nitrogen compounds contained in the fuel and/or air-derived nitrogen entering through system seals is oxidized at high temperature. Additionally, other trace impurities, such as Hg, may form during oxidation. These oxidized compounds of sulfur and nitrogen, which are known to be “acid gases” that are subject to environmental regulations as they are the main catalysts for producing acid rain, may also corrode equipment when present in their aqueous phase, and thus, a need exists to remove and/or maintain the oxidized compounds below certain threshold limits in at least some portions of the power generation cycle. These oxidized components should be removed from the power generation cycle to prevent emission of these toxic impurities to the atmosphere and to protect internal process equipment. Accordingly, combustion-derived gases that produce elevated concentrations of sulfur and nitrogen require post-treatment processing prior to recirculation and/or venting.
While several processes exist for removing sulfur and/or nitrogen from fuels prior to combustion (i.e., pre-combustion removal processes) or for removing trace acid gases from a process gas emitted at the end of the power generation cycle (i.e., post-combustion removal processes), a need exists for a removal process that advantageously utilizes the recirculating design of a power generation cycle, which employs a trans-critical, supercritical, and/or ultra-supercritical working fluid. Such a removal system may advantageously provide for the recycling of CO2 into the power generation cycle at the desired ratios of recycled CO2 concentrations to carbon in the fuel. Such a power generation cycle ideally provides for a controlled low concentration of impurities in the recycled CO2 working fluid stream and/or the product CO2 stream. The impurities may be removed in a form which allows for efficient sustainable disposal and protection of internal equipment. Such a removal process would ideally fill several process needs within a semi-closed loop process, such as cooling, condensing, and removal of pollutants from a recycled working fluid, being relatively inexpensive to build and maintain, having a low parasitic penalty, and employing a simple control and operational strategy.
U.S. Pat. No. 8,580,206 to Allam et al., which is incorporated herein by reference in its entirety, discloses methods of SO2 and/or NOx removal from gaseous CO2 at elevated pressure in the presence of molecular oxygen and water. In particular, a process is provided that utilizes a sequence of gas and/or liquid phase reaction steps where nitric oxide (NO) is oxidized to form nitrogen dioxide (NO2) at an elevated partial pressure of the reactants. This oxidation process may control the overall rate of the reaction sequence. The NO2 then oxidizes sulfur dioxide (SO2) to form sulfur trioxide (SO3), and the NO2 is reduced back to NO. The SO3 then dissolves in the liquid water to form sulfuric acid (H2SO4). The final result is the conversion of SO2 to H2SO4 using NOx as a catalyst. The sequence of reactions is described by the equations listed below.2NO+O2=2NO2  Eq. ASO2+NO2═SO3+NO  Eq. BSO3+H2O═H2SO4  Eq. CExperimental data has confirmed theoretical reaction calculations, which indicate the SO2 concentration can be reduced to very low levels (e.g., below 50 ppm (molar)) in less than 10 seconds when the NOx concentration is above 100 ppm and the pressure is above approximately 10 bar and the oxygen partial pressure is approximately 0.1 bar or higher. See, for example, Murciano, L., White, V., Petrocelli, F., Chadwick, D., “Sour compression process for removal of Sox and NOx from oxyfuel-derived CO2,” Energy Procedia 4 (2011) pp. 908-916; and White, V., Wright, A., Tappe, S., and Yan, J., “The Air Products Vattenfall Oxyfuel CO2 Compression and Purification Pilot Plant at Schwarze Pumpe,” Energy Procedia 37 (2013) 1490-1499, which are incorporated herein by reference in their entirety.
Additionally, U.S. Pat. No. 8,580,206 discloses the use of this known sequence of reactions for removing one or more contaminants, which may include SO2 and/or NOx, in a stream predominantly including CO2 that is provided by an oxy-fuel power boiler that produces steam. In particular, the system includes a pulverized coal fired steam boiler and an oxy-fuel combustion system that recycles substantially all of the flue gas, apart from a net product CO2 rich stream. The flue gas is mixed with the pure O2 oxidant stream and provided to the coal fired combustors, which results in the concentration of NOx being forced to an equilibrium level based at least in part on the combustor adiabatic flame temperature. The combustor adiabatic flame temperature may be high enough to approximately reach near equilibrium conditions for a flue gas NOx concentration. There remains a further need in the art for systems and methods for removing pollutants, particularly acid gases, from output streams in power production.